UV - Visible and¹H or¹³C NMR spectroscopic studies on the specific interaction between lithium ions and the anion from tropolone or 4-isopropyltropolone (hinokitiol) and on the formation of protonated tropolones in acetonitrile or other solvents

The specific interaction between lithium ions and the tropolonate ion (C₇H₅O₂^-: L^-) was examined by means of UV - visible and ¹H or ¹³C NMR spectroscopy in acetonitrile and other solvents. On the basis of the electronic spectra, we can propose the formation of not only coordination-type species (Li⁺(L^-)₂) and the ion pair (Li ⁺L^-) but also a "triple cation" ((Li ⁺)₂L^-) in acetonitrile and acetone; however, no "triple cation" was found in N,N-dimethylformamide (DMF) and in dimethylsulfoxide (DMSO), solvents of higher donicities and only ion pair formation between Li⁺ and L^- in methanol of much higher donicity and acceptivity. The ¹H NMR chemical shifts of the tropolonate ion with increasing Li⁺ concentration verified the formation of (Li⁺)₂L^- species in CD ₃CN and acetone-d₆, but not in DMF-d₆ or CD₃OD. With increasing concentration of LiClO₄ in CD ₃CN, the ¹H NMR signals of 4-isopropyltropolone (HL′) in coexistence with an equivalent amount of Et₃N shifted first toward higher and then toward lower magnetic-fields, which were explained by the formation of (Li⁺)(Et₃NH⁺)L′ ^- and by successive replacement of Et₃NH⁺ with a second Li⁺ to give (Li⁺)₂L′ ^-. In CD₃-CN, the 1,2-C signal in the ¹³C NMR spectrum of tetrabutylammnium tropolonate (n-Bu₄NC₇H ₅O) appeared at an unexpectedly lower magnetic-field (184.4 ppm vs TMS) than that of tropolone (172.7 ppm), while other signals of the tropolonate showed normal shifts toward higher magnetic-fields upon deprotonation from tropolone. Nevertheless, with addition of LiClO₄ at higher concentrations, the higher and lower shifts of magnetic-fields for 1,2-C and other signals, respectively, supported the formation of the (Li ⁺)₂L^- species, which can cause redissolution of LiL precipitates. All of the data with UV - visible and ¹H and ¹³C NMR spectroscopy demonstrated that the protonated tropolone (or the dihydroxytropylium ion), H₂L⁺, was produced by addition of trifluoromethanesulfonic or methanesulfonic acid to tropolone in acetonitrile. The order of the 5-C and 3,7-C signals in ¹³C NMR spectra of the tropolonate ions was altered by addition of less than an equivalent amount of H⁺ to the tropolonate ion in CD₃CN. Theoretical calculations satisfied the experimental ¹³C NMR chemical shift values of L^-, HL, and H₂L⁺ in acetonitrile and were in accordance with the proposed reaction schemes.